380. Insert-Delete-GetRandom-O(1)

difficulty: Medium

Design a data structure that supports all following operations in average O(1) time.

1. insert(val): Inserts an item val to the set if not already present.

2. remove(val): Removes an item val from the set if present.

3. getRandom: Returns a random element from current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();
// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);
// Returns false as 2 does not exist in the set.
randomSet.remove(2);
// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);
// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();
// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);
// 2 was already in the set, so return false.
randomSet.insert(2);
// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

Method One

class RandomizedSet {
    private Map<Integer, Integer> cache;
    private ArrayList<Integer> array;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.cache = new HashMap<Integer, Integer>();
        this.array = new ArrayList<Integer>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if(cache.containsKey(val)) return false;
        int size = array.size();
        cache.put(val, size);
        array.add(size, val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
 // 这里的思想就是把 arralist 中最后一位和 需要被删除的那个位置调换，然后再删除最后一位。这样就避免了 O(N) 的操作，保证了O(1)
    public boolean remove(int val) {
        if(!cache.containsKey(val)) return false;
        int last = array.get(array.size() - 1);
        int index = cache.get(val);
        array.set(index, last);
        cache.put(last, index);
        array.remove(array.size() - 1);
        cache.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int index = (int) Math.floor(Math.random() * array.size() );
        return array.get(index);
    }
}
​
/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */
​